Page 236 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:37 PM Page 199

Heat Transfer 199

above the level of the tubesheet, but this is not Density of vapor 0.0230 lb/ft 3
recommended for differential levels greater than 6 in. Density of liquid 54.5 lb/ft 3
14. After a pressure drop balance has been obtained to Thermal conductivity of liquid 0.084 Btu/hr-ft.-°F
0.1–0.2 psi, compute the heat transfer coefficient as Average c p 0.425 Btu/lb-°F
follows. Average viscosity,
1.3389 c p (2.42)
Shell side: Usual procedure for condensing steam or 3.24 lb/ft-hr
for other heating, medium.
Design: Select a thermosiphon reboiler as the preferred
Tube side: Determine heat transfer coefficient from
operation, if design is acceptable.
Figure 10-46 (using tube-side curve) at Reynold’s num-
ber calculated for pressure drop evaluation. If the h i
2
Assume: flux, Q/A 7,200 Btu/hr (ft )
calculated exceeds 300 for organics (Figure 10-103),
2
U 120 Btu/hr (ft )(°F)
use a value of 300 and correct to outside coefficient, h io .
Max. t 60°F
15. Calculate the overall heat transfer coefficient by
(Steam temp. will be 113°F 60 173°F. This is vacuum steam,
6.417 psia.)
h o h io
U C (10-194)
Approximate surface area:
h io h o
U D = calculated from assumed unit (corrected for final pressure
balance) of Step 7. 1,528,600
A 212 ft 2
7,200
Resistance of fouling and metal tube wall required for
Select: Fixed tube sheet vertical unit
balanced operation of reboiler:
1
1 / 4 -in. tubes 12 BWG steel for low pressure drop, easier
1
U c U D cleaning on 1 / 2 -in. triangular pitch, 4 ft long
r (10-195)
1
U c U D Assume tube sheets, each 1 / 2 in. thick, then usable tube length
3.75 ft.
If the resistance seems too low for the service, then the 212
unit must be redesigned to obtain the higher dirty No. tubes required 2 173 tubes
3.7510.3272 ft >ft2
coefficient, U D .
Shell size estimate:
Other Design Methods
23-in. I.D. shell contains 187 tubes; remove at least 3 for internal
impingement baffle.
Several other excellent presentations exist on the subject
Available area (187 3) (3.75) (0.3272) 226 ft 2
of reboiler design. Presenting all of the variations here is just Recirculation ratio: assume 20:1
not possible; designers should refer to Hughmark, 65, 66, 67 Reboiler vapors required 1,528,600/285 5,370 lb/hr
90
91
Palen and Taborek, Palen and Small, Frank and Prick- Liquid being recirculated 20 (5,370) 107,400 lb/hr
47
ett, and Hajek. 60 Total liquid flow at reboiler inlet 107,400 5370
The article of Fair and Klip 193 presents a detailed analysis 112,770 lb/hr
3
of the necessary design features and equations for horizon- Specific volume of liquid into reboiler 1/54.5 0.01835 ft /lb
3
tal kettle reboilers, horizontal thermosiphon reboilers, and Specific volume of vapor only at outlet 1/0.023 43.5 ft /lb
vertical thermosiphon reboilers. Other useful references on Total volume of mixture out reboiler
3
107.400 (0.01835) 1,980 ft /hr
reboilers are 185, 186, 188, 190, 192, 194, 195, 196, 197, and
5370 (43.5) 233,500 ft /hr
3
201.
3
Total volume 235,480 ft /hr
Specific volume of mixture out reboiler 235,480/112,770
3
2.09 ft /lb
Example 10-21. Vertical Thermosiphon Reboiler,
Kern’s Method 70
Pressure balance across reboiler: Assume fluid in distilla-
tion column at reboiler tubesheet level.
The design of a distillation column requires a reboiler
Static pressure of reboiler leg:
operating at 2.23 psia (vapor space above bottom liquid).
The heat duty is 1,528,600 Btu/hr. The properties of the 2.3L v o
acrylonitrile mixture have been calculated to be 1441v o v i 2 log 10 v i
Latent heat of vaporization 285 Btu/lb 2.3142 log 10 2.09
Average molecular weight 63.4 14412.09 0.018352 0.01835